Diversity receiver



June 5, 1951 Filed Feb. 25, 1947 H. O. PETERSON ET AL DIVERSITY RECEIVER 5 Sheets-She?l 2 vn/meme H. o. PETERSON ET AL 2,555,557

June 5, 1951 DIVERSITY RECEIVER 3 Sheets-Sheet 3 Filed Feb. 25, 1947 Patented June 5, 1951 UNITED STATES PATENT OFFICE DIVERSITY RECEIVER Harold 0. Peterson and Grant E. Hansell, Riverhead, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application February 25, 1947, Serial No. 730,770

(Cl. Z50-8) 2 Claims. 1

This application discloses an improved method of and means for receiving and amplifying telegraphy signals represented by audio tones, the frequency of which is shifted in accordance with signals. The invention is applicable to single channel systems and to high speed systems such as four or eight channel multiplex printer operations, with or without time division multiplexing on the several channels.

The primary object of our invention is an improved method and means for selecting, amplifying, demodulating and relaying for recording purposes, frequency shift tone telegraphy signals which may or may not be multiplexed by time division.

`In frequency shift telegraphy at the transmitter, Wave energy is shifted in accordance with signals from .a first frequency representing mark to a second frequency representing space and .vice versa. The wave frequencies are separated l sufciently to permit segregating the same. The shifted Vwave energy may be of high frequency or of intermediate frequency which is then beat with high frequency oscillations and a side band selected for radiation. At the receiver, diversity effects are made use of to reduce the effects of fading caused by multipath transmission. Several receivers with spaced aerials or aerials having different radiant energy interception characteristics or both are used and means are provided to select signal from that receiver getting the best signal. A system of this type is shown in Schock et al., U. S. Serial #632,978 led December 5, 1945, now Patent #2,515,668, dated July 18, 1950. In `the Said system, the two signal Versions are compared in a differential rectifier and thereby a potential is provided which controls the state of stability of a trigger circuit to turn on that gate tube excited by that receiver having the best signal. Tests show that less printing errors occur when the trigger circuit control potential network has a fast time constant. These time constants were:

Fast=2 milliseconds Medium=,200 milliseconds Slow=2000 milliseconds Testing with a single five-unit printer circuit on signals transmitted from Bolinas (KLR at 10090 kc.) to New York, the following error counts were obtained:

With Fast time constant- 6 errors per 9030 letters With Medium time constant-24 errors per `9030 letters 2 With Slow time constant-52 errors per 9030 letters `at a frequency sufficiently high to permit filtering out the channel ripple without using circuit constants which limit the speed at which channel selecting by switching takes place.

In some installations, a transmitter as used in .telephony sends out a high frequency carrier `amplitudes modulated by a vplurality of tone f requencies each of which is shifted in frequency in accordance with different signals. In other cases, pairsof tones may be shifted in frequency by the same signal and used to modulate `the high frequency carrier. In such a system, the shifted tones are recovered at `the receiver `for recording purposes. We wish to use the im.- proved signal strength sensing and selecting means of the said prior application to select in each channel for `recordingpurposes the best frequency shifted tone. However, where selection is Vrequired at tone frequency, the problem discussed above becomes much more severe.

,If the keyed tones from the `different channels 4of the system are compared as to strength, 4the filter networks necessary to remove the audio ripples would haverelatively slow time constants. The control potential derived by the signal sensing detectors `would be ineffective to operate the triggering circuit for gate control purposes fast `enough it@ select mar characters from one `channel and space characters from the other channel.

An object of our present invention is to provide improved means for selecting from -tWo or more keyed tones, which are different versionsof the same signal such as might be obtained in the detector `outputs of single or diversied receivers, that tone Iwhich is of the best lstrength and to do so at a rate sufficiently high vto select mark characters from one channeliand space characters from the other channel.

`In describing our inventionin detail, reference v Same 1113.111181' output. In the embodiment of Fig. 3, receivers l in triple space diversity are used to derive the keyed tones.

In one system of multiplex operation, a number of different tone channels are transmitted by a radio telephone transmitter which may be of either the double side band or single side band variety using either amplitude modulation or phase modulation in the double side band transmitter with receivers provided to accommodate whatever form of modulation is being transmitted. For reception, a plurality of receivers are provided, connected to independent spaced antennas. VThe receivers may be of the dual diversity type, and in fact, may be substantially as illustrated in Peterson U. S. application,

Serial #634,350, nled December 11, 1945. The transmitted wave might comprise a carrier on which a plurality of tones are modulated, the tone frequencies being shifted in accordance with signals to represent mark and space the drawings and in the above mentioned application, it is assumed that at least four channels of which three are shown, are used so that at yleast four tones shifted in frequency between mark and space are derived at the receiver outputs. "The receivers of rectangles I@ and ID may then be about as illustrated in Fig. l of the drawings of said application and may include all of the apparatus illustrated and other elements Yknown in the art to supply from detectors to lines 36 and 36 corresponding to lines 36 and 36 of the said application, a plurality of tones, the frequencies of which shift in accordance with signals. In an example given, channel l might operate at 1450 cycles, 180 cycles. Channel 2 might operate at 1650 cycles i80 cycles 4and channel 3 might operate at 1850 cycles m80 cycles. Furthermore, time division multiplexing may take place on veach channel so that a large number of printer circuits may be handled by the several tone channels.

Each audio tone channel is frequency shift modulated by telegraph signals which may, for instance, be four channel time division printer signals. These different tone channels are selected by band pass filters 130, 42, 40 and 42',

etc. in the outputs of each of the diversity receivers. In the drawings, for the sake of simplicity, we have shown two band pass filters coupled to the output of each receiver. The additional band pass filters are connected in the The remaining apparatus in the figure is the improved means of the present invention for yprocessing the tone signals derived at the receiver outputs in such a manner as to use that signal from that receiver in which the particular tone frequency is strongest at any moment.

'I'o obtain the benefits of space diversity reception by a receiver as illustrated in Fig. 1, means must be provided for selecting tone output from that receiver having the strongest tone at the moment. Furthermore, it has been found by measurements that best results are obtained -if `a very fast switching device is used so that,

4 for instance, the mark signal may be taken from one receiver and the space signal from the other receiver. This has been explained above and means that the switching control must operate in a length of time which is short compared to the length of one band. When 4-channel time division multiplex is being transmitted the length of one band is approximately six milliseconds and it is therefore desirable for the switching operation to function completely in `less than one millisecond. A signal strength sensing and receiver output switching means suiciently fast to operate at this required speed has been disclosed in Schock et al. patent previously mentioned. Y

If we use rectified tone voltage for controlling the switching operation, we find it difficult to filter off the tone frequency ripple, and yet have a time constant in the gate control network which is on the order of one millisecond.

It is easier to filter off from the rectifier output the ripple frequency if the shifted tone is of higher frequency and, therefore, in accordance with our invention, we propose to heterodyne the tone frequency up to a higher frequency, say for exam-ple, on the order of 50 kc. Then the signal strength sensing circuits and connections may have filters sufficiently slow to lter off the intermediate frequency ripple and still fast enough to select the strongest signal, even if the switching operation needs to be so fast that mark is selected in one receiver and space from the other receiver.

In the drawings, a. local oscillator 50 supplies output to two converters or modulators 52 and 52. Additional converters, not shown, are .supplied for the other channels of the system. The oscillations of source 5i) are mixed with the frequency shifted tones in the converters 52, 52 to supply higher frequency outputs to lters 58, 58'. Filters 53, 58 maybe of the band pass type and the modulated frequency shifted energies are supplied to signal strength sensing and detecting means cooperating with signal gating means to select signals from the channel having the strongest signal. In the example given above the filters 5S, 58 etc. may have a pass band of 50 kc. i500 c/sec. or less. The modulators 52 and 52' may be of the balanced carrier suppressor modulator type or any other suitable type. The upper or lower side band may be selected by the filters 58 and 58.

Any signal strength sensing and selecting means with signal gating means controlled thereby may be used. In the embodiment illustrated, we prefer to use a signal strength sensing and detecting arrangement as shown in the said Schock et al. patent, for producing a control potential to operate triggering circuits in turn controlling gating tubes, all as disclosed in said patent. I'he gate control circuit of said patent operates at 50 kc. and can be made suiiiciently fast to'switch at signalling speeds. Rectangle 60 is then to include signal strength sensing means for deriving the gating control potential, the trigger control tube, the triggeringV circuit in unit 6I, the gating tubes 63'and 63 etc., comprising elements included in the dotted rectangle of Figure 1 of said patent or so much Vthereof as is needed to supply the keyed output. The output of the gating tubes may be of signal frequency, assuming that detection takes place before the signals are applied to the gate tubes, as shown here and in said application,V

or the output of the gating tubes may be of Vintermediate frequency, in which case gating takes place before detection and a detector follows the gate tubes. The units 53 and 53 include the `discriminators and detectors. lIhe essential feature, however, is to raise the frequency of the keyed tones to a higher frequency before subjecting the same to the signal sensing detection operation.

The resulting `keyed signal may be used directly for recording purposes, or may be supplied to a tone keyer ll which in turn operates a recording means such. as a teletypewriteiz The tone lkeyer may be as illustrated inPeterson et al., U. S. application Serial #630,.:23, :filed -l\lcvember 7, 1945, nowabandoned.

Our invention is also applicable to systems wherein -frequency diversity only is made use of. For example, two tones of different frequencies are frequency shifted in accordance with the same signal and the several tones are then used to amplitude modulate a carrier. Then a single receiver system as shown in Figure 2 may be used. The receiver may be like that at lil in Figure l, supplying as its detector output the several frequency shifted tones. For example, the keyed tones might be of i000 cycles i8@ cycles, 1300 cycles i8@ cycles etc. used to isolate or separate out the various tones, and the two tones keyed by the same signal are converted, in accordance with our invention, to higher 'frequencies and then applied to the signal strength selecting means. Separate oscillators 5G and lill -are now used and may operate at 48,700 and 48,400 cycles/sec. respectively. The band pass filters 58 and 58 select the frequency shifted signals of higher frequency and feed them to a signal selector and frequency shift detector 6l), which may be like the apparatus in unit til of Figure 1. The detected output may again be fed to a tone keyer dii. The operation is as described in connection with Figure 1. Other frequency converters are connected in like manner to the other pairs of tone f lters passing tone frequency waves keyed by the same signals.

Qur invention is not limited to the use of dii versity systems of the type wherein two versions of the signal only are used. On the contrary, we propose to use our system with, receivers in triple diversity. Furthermore, the signal strength sensing circuits in the rectifiers in which is produced the control potential for diversity switching or selecting, may be of any appropriate type, various ones of which are known in the art. For example, our invention may be applied to triple diversity systems using three receivers, somewhat as illustrated in Peterson Patent #2,290,992. Moreover, gating may be accomplished somewhat as illustrated in Figure 3 of said patent. Such a system including our invention, would then be substantially as disclosed in Figure 3 of the ,drawings. In this figure, there are three receivers Il), lil and it, each including an aerial or other signal pick-up means, radio frequency amplifiers, a source of oscillations one source for each receiver but preferably a common source for all of the receivers) and converting detectors wherein the received signal is reduced in frequency to an intermediate frequency. Said receivers may also each include an intermediate frequency amplier and a detector with common automatic gain control circuits, if desired. The receivers here may be as illustrated in the said Peterson patent or in said Peterson application, Serial #634,350 filed December 11, 1945. The various tone frequencies in the multiplex transmitting Filters at, 32, etc. are.,

converter modulators 52 52 and 52 system might be impressed as amplitude modulation on a carrier, Ain which case the receiver has an amplitude modulation detector in the output of which the various keyed tones appear. These frequencyshifted tones appear in lines 36, `35 and 36 for selection by filters, asin the arrangement of Figure l. In the arrangement of Figure 3, a single lter is shown connected with each of the lines 3.6, 35 and 36". These band pass filters have been designated 4U, 4t' and 40".. The selected tone frequencies are impressed on also eX- cited by oscillations from the oscillation source iid. The outputs from the converters are impressed on selective circuits 5E, 55 and 5S which include band pass filters, tuned to the upper or lower side frequencies, amplifiers and frequency shift detectors. The detectors convert the frequency shifts in tone frequency into correspondingly varying potentials having a strong direct current component. These potentials are fed by way of capacitors 6I, Si and Si and coupling resistors to the control grid of a coupling tube S5. The capacitors 6l, El and 6l" remove the direct current components, to supply alternating potentials to the control grid of the tube The tube t5 has its cathode connected to ground by a resistor and has a grid biasing resistor GR which operates to supply output from that detector only to which is being fed the best signal at that particular instant. To do this, the resistor GR supplies to the control grid of tube 55 a high negativepotential, so that the same is biased to cut-olf for all applied potentials except that signal potential from that one of the three receivers having the greatest magnitude. For example, if there is a strong signal supplied by detector 56 and weaker signals by the other detectors, the negative potential supplied by GR. to the grid of tube uit is high enough so that this tube remains at cut-off with respect to the weaker signals. However, the stronger signal from detector 5E overcomes this negative bias so that the tube 55 responds to the signal at that instant, causing current to flow in the tube. Ii at the next instant, this signal in 56 becomes weaker and that supplied by detector 56 'becomes stronger, the latter signal takes over because the signal in the current amplitude limiter of this channel is strong enough to produce limiting action and takes over to supply tube t5 and makes its grid potential more negative to cut oif signal from the weaker channels. In other words, the three input levels to the tube 65 are adjusted such that they are only slightly above the level necessary to produce limiting in the current amplitude limiters feeding the frequency shift detectors. Then, as a given signal fades, it will drop below the level required for limiting in that channel and the output of the discriminator detector in that channel will drop until that set is not contributing appreciably to the output. The selected output appears in the load resistor LR, in the anode circuit of tube 65 and is fed through a low pass filter 51 to a tone keyer 8D and thence to recording means.

What is claimed is:

1. In a frequency shift telegraphy system in which the mark element of a signal is characterized by one frequency and the space element of the same signal is characterized by a different frequency, means for receiving several versions of said signal, said means including means for converting the frequencies of the mark and space elements of the received signal to different tone frequencies inthe audio range, so that each signal version is represented by a tone of one frequency for marl; and a tone of a dierent frequency for space, means coupled to the output of said converting means for heterodyning the tone frequencies to corresponding waves of higher frequency, strength comparing means to which the several higher frequency waves are applied, said `comparing means including rectifiers for rectifying the waves applied thereto, and lters coupled to the outputs of said rectiers for removing -ripple resulting from the rectication, said lters having Ysuch time constants as to enable fast switching between signal versions.

2. In a frequency shift telegraphy system in which the-mark element of a signal is characterized by one frequency and the space element of the same signal is characterized by a different frequency, means for receiving several versions of said signal, said means including means for converting the frequencies of the mark and space elements of the received signal to different tone frequencies in the audio range, so that each signal version is represented by a tone of one frequency for mark and a tone of a different frequency for space, means coupled to the output of said converting means for heterodyning the tone frequencies to corresponding waves of higher frequency, strength comparing means to which the several higher frequency waves are applied, said comparing means including rectiers for rectifyingrthe waves applied thereto, lters coupled to the outputs of said rectiers for removing ripple resulting from the rectification, said lters having such time constants as to enable fast switching between signal versions, said comparing means functioning to derive a control potential the absolute magnitude of which depends on 8 Y A\ which of the several waves of higher frequency represents the best version of the signal, a plurality of gating devices, equal in number to the number of signal versions being received, controlled by and responsive to said potential to pass the signal version impressed thereon, means for impressing a diierent signal version on each gating device, and a common output circuit coupled to said gating devices. Y Y HAROLD O. PETERSON. GRANT E. HANSELL.

REFERENCES CITED The following references are or" record in the 111e of this patent:

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